1. Field of the Invention
The present invention relates to a ball screw device and a steering system.
2. Description of the Related Art
As a conventional electric power steering system (EPS), an EPS is known that uses a ball screw device to convert rotational torque of a motor into a linear motion of a rack shaft, thereby assisting a driver in steering operation.
In such a ball screw device, a ball screw nut that is rotated by a motor is screwed onto a rack shaft via balls. In conjunction with rotation of the ball screw nut, the balls roll while rotating in the same direction as the ball screw nut rotates, between the ball screw nut and the rack shaft. If adjacent balls come into contact with each other during rolling of the balls, sliding resistance is generated between the adjacent balls, which may cause the rotational torque of the ball screw device to fluctuate. In view of this, a ball screw device of a steering system described in Japanese Patent Application Publication No. 2009-257369 is provided with a cylindrical retainer (cage) having retainer grooves that retain balls in a rollable manner in order to prevent adjacent balls from coming into contact with each other. Inner surfaces of each retainer groove that face each other in the circumferential direction are inclined so as to be separated from each other toward the radial outside of the retainer groove. Thus, movement of the balls toward the radial outside of the retainer grooves is allowed, but movement thereof toward the radial inside of the retainer grooves is restricted.
In order to facilitate processing of the retainer grooves, the angle of inclination of the inner surfaces of each retainer groove is desired to be smaller. However, when the angle of inclination of the inner surfaces of each retainer groove is set to be smaller, time and labor to process the inner surfaces of the retainer grooves can be reduced, but it is difficult for the balls to ride on the retainer, and it is difficult for the retainer to retain the balls in the radial direction of the retainer. The reason for the reduction of time and labor to process the inner surfaces of the retainer grooves is that a smaller inclination angle of the inner surfaces of the retainer grooves requires fewer machining processes such as cutting of inclined surfaces thereof.